To avoid some drawbacks with older binder apparatus technique such as uncertainty in judging the length of time to be set for attaining a proper binding, a new technique was presented in the U.S. Pat. No. 4,367,116 "Binding Apparatus". By this technique the heating element was supplied continuously with power for maintaining a desired working temperature. Since the binding apparatus described in the patent was formed to cater for folders with a different number of pages by having an adjustable opening there was a demand for different operation times which were dependent on the actual folder thickness. This was solved by having a set of buttons for pre-setting the length of the operating time.
The known binding apparatus according to the mentioned U.S. patent is indeed simple to operate but relatively energy-demanding since it operates with a continuous power supply.
It has also been proposed to have intermittent power supply of the heating element in a binding apparatus, for example in the German patent 3 514 201. In this case a heat detector is arranged for sensing the temperature of the heating element, the power supply to which is interrupted after each binding operation. The heat detector output signal is used to control the start of the time switch so that the pre-set operating time is counted from and including the time when the heating element has reached its pre-determined working temperature, stated to be at least 90.degree. C.
There has indeed been achieved that the power consumption is reduced to a certain extent, but the power consumption is still unnecessary, since it has been found during undertaken investigations that the binding operation can also utilize the time when the heating element has a lower temperature. Accordingly, the energy consumption could be brought to an absolute minimum if the time count on the time switch could be caused to take into account what starting temperature the heating element has when the folder is inserted in the opening of the binding apparatus. A basic condition is also to get the time count of the time switch to take into account the set width of the opening. This is achieved by the present invention.